Nitro-substituted aromatic acid corrosion inhibitors for alkanolamine gas treating system

ABSTRACT

Corrosion of metallic surfaces by aqueous alkanolamine solutions employed in acid gas removal service can be inhibited by combinations of antimony and vanadium compounds, stannous salts, organo-tin compounds, nitro aromatic acids and their salts or benzotriazole.

United States Patent 1 Mago et al.

NITRO-SUBSTITUTED AROMATIC ACID CORROSION INHIBITORS FOR ALKANOLAMINEGAS TREATING SYSTEM Inventors: Blake F. Mago, New City; Charles W. West,Niagara Falls, both of Assignee: Union Carbide Corporation, New

York, NY.

Filed: Nov. 2, 1973 Appl. No.: 412,508

Related US. Application Data Division of Ser. No. 201,131, Nov. 22,1971, Pat. No. 3,808,140, which is a continuation-in-part of Ser. No.54,595, July 13, 1970, abandoned.

US. Cl. 252/192; 252/189; 252/392; 423/229; 21/27 R Int. Cl. BOld 53/34;C23f 11/14 Field of Search 252/389 R, 387, 192, 189, 252/390, 392;423/228, 229; 21/27 R [451 July 22,1975

References Cited OTHER PUBLICATIONS Rosenfeld et al., Mechanism of MetalProtection by Volatile Inhibitors, Corrosion, Vol. 20, 1964, pp.222t234t.

Primary Examiner-Benjamin R. Padgett 'Assistant Examiner-Irwin GluckAttorney, Agent, or Firm-S. R. Bresch 10 Claims, No Drawings BACKGROUNDOF THE INVENTION This invention relates to novel corrosion inhibitorsfor alkanolamine-gas treating systems.

Gases such as natural gas, flue gas and synthesis gas have been purifiedby the utilization of aqueous alkanolamine solutions for the absorptionof acid gases such as CO H 8 and COS contained in the gas stream.Ordinarily, a percent to 30 percent alkanolamine solution (e.g., amonoethanolamine solution) flows counter current to the gas stream in anabsorption column in order to remove the acid gases. An advantage ofsuch a system is that the process is a continuous cyclic one and thereaction can thus be reversed at higher temperatures in order toliberate the acid gases from the solution.

When steel parts or components are used in such a system, it has beenfound that both general and local corrosive attack can occur. This is aparticular problem in reboilers and heat exchangers where the steel isexposed to a hot, protonated alkanolamine solution. A heat transferringmetal surface appears to be especially vulnerable. Previousinvestigation by others have revelaed that under certain conditionscorrosive products such as aminoacetic, glycolic, oxalic and formicacids were formed. The monoethanolamine salts of these acids present thepossibility of increased attack upon ferrous metals. Furthermore, thecarbonate salt of monoehanolamine can be converted to additionalproducts such as N(2-hydroxyethyl) ethylenediamine which has been foundto increase corrosivity towards steel, particularly under heat transferconditions.

There are various alternatives available in order to decrease corrosionrates, among them (1) the provision of a side-stream reclaimer to removecorrosive degradation products, (2) the employment of more corrosiveresistant construction materials, (3) greater control of the processconditions and (4) the inclusion of corrosion inhibitors. From both costand efficiency standpoints, the last alternative is preferred. However,certain corrosion inhibitors indicated to be effective have not gainedindustry wide acceptance possibly because of an inability to providecontinuing protection in certain respects.

SUMMARY OF THE INVENTION It has now been found that the corrosion ofmetallic surfaces by aqueous alkanolamine solutions employed in acid gasremoval service can be inhibited by an inhibiting amount of corrosioninhibitors including combinations of antimony compounds and vanadiumcompounds which are at least partially soluble in said aqueousalkanolamine solutions, stannous salts, organo-tin compounds, nitroaromatic acids and their salts and benzotriazole. The corrosioninhibitors described herein are especially useful in the aqueousmonoethanolamine scrubbers employed in ammonia plant systems for theproduction of hydrogen.

Antimony compounds have been used previously as inhibitors forpreventing attack of ferrous metals by aqueous monoethanolaminesolutions. One hypothesis of those who have previously worked withantimonycontaining compounds in acid solutions is that they function asiron or steel inhibitors by being reduced to form a deposit of antimonyon the metal surface, and that inhibition results from its relativelyhigh hydrogen overvoltage or increased polarization of local actioncathodes. There is also the possibility of a secondary anodiccontribution as well.

Vanadates have been known in the past to be iron or steel corrosioninhibitors but have not been utilized widely for this purpose. Theoxidation states of vanadium would suggest that the vanadates mayfunction as oxidant-type inhibitors.

It has also been found that in spite of the differences in inhibitingmechanisms by antimony and vanadium compounds, the combination of thetwo additives is surprisingly superior to each one alone at the sameconcentration.

The term partially soluble as used in this invention is intended to meansolubilities as low as about 0.01 grams per ml. of aqueous alkanolaminesolutions employed in acid gas removal service.

The choice of vanadium compounds is not critical since it is thevanadium-containing anion particularly vanadium in the plus 4 or 5valence state, which provides this unusual corrosion inhibiting propertyin combination with antimony ions. Thus, for example, one can employvanadium oxide such as VO, V 0 V 0 and the like; vanadium cyanides suchas, K V(CN) .3H O, K V(CN) 2KSCN'VO(SCN) 51-1 0 and the like; vanadiumhalides, such as, fluorides, including VF VF .3H O, VF VOF VF or VOFchlorides including VCI VCI VCI .6H O, VOCl, VOCl VOCl V O Cl, V O Cl.4I-l O or VO Cl- .8- H O, bromides including VBr VBr .6H O, VOBr, VOBror VOBr and iodides including V1 VI .6I-I O or V1 vanadium sulfatesincluding VSO .7H O, V (SO VOSO, or (VO) (SO vanadates includingorthovanadates, represented by the generic formula: M VO pyro vanadates,represented by the general formula M V O and metavanadates, representedby the general formula MVO and the like where M represents a cation. Thecondensed vanadate ions which form in aqueous solutions, such as, V 0-are also useful in this invention.

For convenience in introducing vanadate ions into the inhibiting systemsof this invention the alkali metals, ammonium and alkaline earthvanadates including orthovanadates, pyrovanadates and metavanadates arepreferred. Exemplary of such vanadates are the following: sodiummetavanadate, potassium metavanadate, lithium metavanadate, ammoniummetavanadate, sodium pyrovanadate, potassium pyrovanadate, lithiumpyrovanadate, ammonium pyrovanadate, sodium orthoyanadate, potassiumorthovanadate, lithium orthovanadate, ammonium orthovanadate, calciumorthovanadate, calcium pyrovanadate, calcium metavanadate, magnesiumorthovanadate, magnesium pyrovanadate, magnesium metavanadate, ferrousorthovanadate, ferrous pyrovanadate, ferrous metavanadate, copperorthovanadate, copper pyrovanadate, copper metavanadate, and the like.

Other forms of vanadium that can be used in this invention include: thevanadovanadates, double vanadates, i.e., heteropoly acids containingvanadium and the peroxy vanadates having the general formula:

MVO The preferred cations represented by M are the alkali metal andammonium cations.

The preferred antimony compounds used in this invention are: antimonylcompounds, such as, alkali metal antimonyl tartrates, alkali metalantimonyl gluconates and other such antimony derivatives of polyhydroxyorganic aicds, wherein the aliphatic carboxylic acid moiety has fromabout 2 to about 6 carbon atoms. A preferred antimonyl compound ispotassium antimonyl tartrate having the formula: K(SbOH )C H O /2H O aswell as sodium antimonyl tartrates. When alkali metal antimonyltartrates are used in the combination of the instant invention, smallamounts of tartaric acid, that is about 1.0 percent to about 50 percentby weight of the antimony compound is also pref erably employed forimproved astability.

Other antimony compounds which can be used in the process of thisinvention include antimony trioxide or pentoxide reaction products withorthodihydric phenols, sugar alcohols, and similar hydroxy compoundswhich form definite but complex compounds.

Additionalantimonyl compounds which can be used in this inventioninclude oxides of antimony such as antimony trioxide, Sb O antimonytetroxide, Sb O antimony pentoxide, Sb O alkali metal metaantimonites,and pyro-antimonates and metaantimonates, antimony sulfates, and thelike.

For convenience in introducing the antimony compounds into the aqueousalkanolamine solutions, it is preferred to employ them in conjunctionwith solubilizing or chelating agents, such as, tartaric acid, ethylenediamine tetraacetic acid, and the like.

Still another group of antimony compounds which can be used areantimony-carbon compounds, i.e., organometallic compounds of antimony.These are exemplified by the arylstibonic acids having a generic formulaArSbO H- where Ar represents an aryl group. Specific examples includepara-aminobenzene stibonic acid, p-NH C l-hSbO H para-diethylaminobenzene stibamine, para-acetaminobenzene stibonic acid and its alkalimetals, para-stibosoacetanilide, OSbC I-LNH- COCH and the like.

In using the antimony and vanadium compounds of this invention therespective compounds are mixed together such that there is a ratio offrom about 1 to about 9 parts by weight, of antimony compound to about 9to about 1 part by weight of vanadium compound. The preferred ratios arefrom about 4-6 parts to about 6-4 parts with equal parts being mostpreferred.

The combination of antimony and vanadium compounds is added in an amountof from about 0.01 to about 2.0 percent by weight based on the weight ofthe aqueous alkanolamine solutions including the weight of the water andthe alkanolamine. These percentages apply to all of the corrosioninhibitors encompassed by the instant invention.

Another class of compounds which have been found useful as corrosioninhibitors for aqueous alkanolamine systems are tin compounds such asstannous salts exemplified by stannous tartrate, stannous gluconate,stannous chloride, stannous acetate, stannous fluoborate, and organo-tincompounds such as di-n-butyltin dimethoxide, n-butylstannoic acid,dimethyltin oxide and diethyltin dichloride, Stannous tartrate andstannous gluconate are preferred with stannous tartrate especiallypreferred.

Stannous tartrate and stannous gluconate are preferred compounds forthey have been found to be soluble in amounts of at least 1 percent byweight and up to about 2 percent by weight in concentratedalkanolamines. For example, monoethanolamine can be sold as an inhibitedproduct containing up to about 2 percent by weight of stannous tartrateand/or stannous gluconate making it easier to formulate an aqueousalkanolamine purification system at a plant.

Additional corrosion inhibitors are the nitrosubstituted aromatic acidsand their salts such as sodium-nitrobenzoate, sodium 4-nitrophthalate,pnitrocinnamic acid and the like. These compounds may be oxidant-typeinhibitors caused by effecting some anodic polarization, the mechanismperhaps involving chemisorption along with oxidation. Protection usingthese compounds as inhibitors tends to be relatively de pendent upontemperature.

A further corrosion inhibitor for the aqueous alkanolamine systemscomprising a part of the instant invention is benzotriazole. It has notyet been determined whether benzotriazole operates as an anodicinhibitor or also involves a significant cathodic contribution in thiscase.

Alkanolamine systems which are benefitted by the inclusion of theinstant corrosion inhibitors are those mono and polyalkanolamines havingfrom 2 m4 carbon atoms per alkanol moiety. Typical alkanolamines aremonoethanolamine, diethanolamine, and monoisopropanolamine.

The corrison inhibitors of the instant invention were tested inmonoethanolamine-water-carbon dioxide solutions because while aqueousmonoethanolamine solutions by themselves are not corrosive towardsferrous metals, when saturated with carbon dioxide they become quitecorrosive to mild steel. It is thought that electrochemical corrosion isinvolved with the anodic reaction expected to produce products such asferrous hydroxide, ferrous carbonate or certain complexes.

In some of the examples, metal strips 3 inches 1 /2 inches X 1 I16inches were cleaned by scrubbing with a bristle brush employing a mildabrasive, followed by rinsing with water and acetone. The dry, cleanmetal strips were then weighed and placed upright in a 600 ml. glasscell, after which the strips were separated by means of Z-shaped glassrods. The strips were covered by adding 400 milliliters of themonoethanolamine test solution that had previously been loaded at roomtem" perature with carbon dioxide. Each cell was then fitted with areflux condenser, a sparging tube and a thermometer, and placed in aconstant temperature bath. The solution was maintained at the testtemperature for 72 hours while bubbling with carbon dioxide at astandard rate of cc/min. The metal panels were cleaned by immersion inan inhibited hydrochloric acid solution for a short time, rinsing inwater and acetone, and air drying. Weight loss was then determined.

Heat transfer effects relative to the corrosion of steel were measuredas follows: A weighed steel plate (3inches 3inches X 3/ 16inch) wassecured by means of a two-inch pipe joint arrangement to a 1000milliliter flask. The plate was heated with a SOO-watt soldering ironfor which a special head had been made in order to lock the unittogether. A Variac was employed to control the heat input and athermocouple well was drilled half-way in from the edge of the plate torecord the approximate metal temperature. The flask was fitted with areflux condenser, a thermometer, and a sparging tube. In all tests theheat input was sufficient to maintain a vigorous boiling for the 72 hourperiod while bubbling with carbon dioxide at a standard rate of 100cc./min. To compare the effect of heating steel by immersion to that ofhaving it the heating source, a 1 /zinch /2inch l/l6inch panel of thesame steel was suspended by a hook in the solution. In dilutemonoethanolamine solutions, the corrosion rate of the heat maintainingvigorous boiling of the solution. A thermocouple reading indicated thatthe heat transfer plate temperature was about 120C. for a 15 percentaqueous monoethanolamine solution whose bulk temperature was about 101C.Test duration was 72 hours for all experiments with carbon dioxidesparging. Percent protection was calculated as follows:

(weight loss uninhibited weight loss inhibited)/ 100 transfer plate wasfound to be significantly greater than 10 (weight loss uninhibited) 100The results were as follows: Protection of Mild Steel,

Inhibitor Heat Sus- Appearance of and Amine Transfer pended HeatTransfer Amount Solution Plate Panel Plate After Test Good-possibly 0.1%Sodium MEA 85 85 few pits metavanadate MBA 94 95 Like new 15% HEED 0 0Severe crevice pits accounting for weight loss 0.1% Potassium antimonylSlight general tartrate and 30% MBA 83 80 attack 0.01% Tartari 15% HEED95 28 Like new Acid 0.05% Sodium 15% MBA 93 92 Like new metavanadate, I0.05% potas- 30% MBA 96 97 Like new sium antimonyl tartrate, and 0.005%15% HEED 97 90 Like new tartaric acid 'Monoethanolan'iineN(2-hydroxyethyl)ethylenediamine that of the immersed panel. The metalplates and pan EXAMPLE 2 els were cleaned after testing by immersion inan inhibited hydrochloric acid solution for a short time, rinsed inwater and acetone, and air dried. Corrosion of steel was determined byboth weight losses and appearance.

The corrosion of both mild steel and 304 stainless steel bymonoethanolamine solutions under conditions of higher temperatures andpressures was studied using a Parr Series 4500 pressure reactor. Cleanand weighed metal panels 3inch X Ainch 1/16 inch, suspended by hooksfrom a glass liner in the reactor, were completely covered by themonoethanolamine solution that had been treated with carbon dioxide atroom temperature. After closing the reactor with its pressure head,carbon dioxide was bubbled through the solution to reduce oxygenavailability. The unit was then heated at the desired temperature for 24hours under the natural pressure developed by the solution. After this,the metal panels were cleaned by immersion for a short time in aninhibited hydrochloric acid solution, rinsing with water and acetone,and air drying.

The previously described test procedures were used in the followingExamples which are representative of this invention.

All parts and percentages are by weight unless otherwise specified.

EXAMPLE 1 In this example the equipment was designed such that the steelplate in question was also the heat source for Vanadium Compound Addedwith Like Concentration of Potassium Antimony] Tartrate to 30% PercentProtection of MEAH OCO Solution Mild Steel Heat Transfer Suspended PlatePanel 0.0375% Vanadium Pentoxide 92% 93% 0.195% Sodium orthovanadatehexadecylhydrate 98% 94% 0.062% Sodium pyrovanadate 93% 93% 0.044%Sodium tetravanadate 91% 94% *0.05% Potassium Antimony] Tartrate and0.005% Tartaric Acid EXAMPLE 3 The lack of criticality of the form ofvanadium used in formulating the combination of vanadium and antimonycompounds was demonstrated by using mixtures of vanadium pentoxide andaqueous caustic plus hydrogen peroxide or sodium peroxide and by mixingsodium decavanadate with aqueous sodium hydroxide. The

corrosion test of these mixtures are shown below:

Vanadate-Containing Solution Added with 0.03% Potassium AntimonylTartrate* to a 20% MEA-H OCO Solution Percent Protection of Mild SteelHeat Transfer Suspended Plate Panel Equivalent to 0.035% NaVO preparedby mixing a high purity V into water containing a stoichiometric amountof NaOH plus a small amount of H 0 Equivalent to 0.035% NaVO prepared bymixing a high purity V0 O into water containing a stoichiometric amountof NaOH and Na- O Equivalent to 0.035% NaVO prepared by mixing sodiumdecavanadate into water containing a stoichiometric amount of NaOH.

0.035% NaVO (Foote Mineral Company) Plus 0.003% Tartaric Acid EXAMPLE 4When combinations of sodium metavanadate with a number of antimonycompounds were used in corrosion protection experiments as previouslydescribed results similar to those described in the prior examples wereobtained. These antimony compounds included antimony tartrate, antimonylactate, sodium antimony tartrate with tartaric acid, and antimonypentachloride. The corrosion data obtained with mild steel test panelsare shown in the table below together with the relativeconcentrations-of the corrosion inhibitors.

Antimony Compounds Added with Like Concentration of Sodium PercentProtection of Metavanadate to 30% Aqueous Mild Steel MEA SolutionSparged with CO, Heat Transfer Suspended Plate Panel 0.036% AntimonyTartrate 90% 95% 0.0l83% Antimony Lactate 89% 97% 0.030% Tartar Emetic0.003% Tartaric Acid 88% 94% 0.0285% Sodium Antimony] Tartrate .0029%Tartaric Acid 95% 93% 0.0270% Antimony Pentachloride 94% 92%Concentrations were selected to provide 0.0l 1% antimony in each testrun.

0.035% Sodium Metavanadate Calculated by formula:

Wt. Loss of Uninhibited Wt. Loss of Inhibited EXAMPLE 5 EvaluationConditions Uninhibited Inhibited Temperature, C. 100 I25 I00 I25Pressure Reading, psi I20 200 l 250 Time, hours 24 24 24 24 5 CorrosionLosses,

mils per year Cold-rolled mild steel 20 1 1 I 304 Stainless steel 1 1nil nil EXAMPLE 6 To test the effect of the corrosion inhibitors of theinstant invention under more severe conditions, the antimony-vanadatecombination was compared to the individual additives in corrosion testswith aqueous monoethanolamine solutions containing 65 weight percentmonoethanolamine at the boiling point of the solutions that werecontinuously sparged with carbon dioxide. The purpose of theseconditions is that they offer a more rapid and severe means forevaluating inhibitors. After 72 hours under these conditions, corrosionof steel panels was determined as in the previous examples by comparingweight losses to similar panels immersed in uninhibited solutions. It isapparent from the following results with duplicate and sometimestriplicate experiments that reproducibly satisfactory protection wasrealized with the combination but not with the individual additives.

Inhibitor and Amount,

Range of Percent alone is characteristic of anodic inhibitors at aborderline concentration and sometimes this is evidenced by severelocalized attack.

EXAMPLE 7 In this example, one of the tin compounds of the instantinvention, stannous tartrate, was evaluated at various concentrationsunder heat transfer conditions for its protection of mild steel in twodifferent systems made up of monoethanolamine, water, carbon dioxide andstannous tartrate. All of the solutions were constantly sparged withcarbon dioxide. The results were as follows:

Test Weight Percent of Stannous Percent Tartrate Protection PercentProtection of Immersed Mild Steel Panels calculated by:

Wt. Loss Uninhibited Wt. Loss Inhibited Wt. Loss Uninhibited AIOO "Asolution containing 15 weight percent monoethanolamine heated atapproximately 101C. for three days. "A solution containing 65 weightpercent monoethanolamine heated at approximately 107C. for three days.

The following example was undertaken in order to test the sodiummetavanadate, potassium antimonyl tartrate and tartaric acid inhibitorsystem under actual plant conditions.

EXAMPLE 8 The same plant was inspected further during a turn aroundperiod for evidences of corrosion during the five months periodfollowing inhibitor addition. The tube side of the lean-rich heatexchanger was examined since corrosion, particularly of the baffleplate, had been a problem with the uninhibited amine solution. Afterfive months of operation with the inhibitor, no attack was evident onany of the component parts. Moreover, the top sections of the strippercolumns and manifold lines to the tube side of the heat exchanger thathad frequently been perforated by the uninhibited solution did notdevelop any leaks during the period of operation with the inhibitorcombination.

EXAMPLE 9 Using the same techniques as described in the Example 4,additional species of tin compounds were evaluated as to their abilityto inhibit corrosion of steel under heat transfer conditions. The testrevealed the following information:

Tin Amine Percent Protection Compound, Solu- Heat Sus- Weight tionTranspended fer Plate Panel 0.025% stannous tartrate 15% MI'ZA 75 800.025% stannous gluconate 75 80 0.05% stannous gluconate MEA 80 90 0.05%stannous tartrate 90 95 0.05% stannous chloride 85 90 0.05% stannousacetate 70 90 0.05% stannous fluoborate 70 85 0.1% potassium stannate 00 0.05% stannic chloride 0 O 0.05% di-n-butyltin dimethoxide 85 85 0.05%n-butylstannoic acid 45 70 0.05% stannous tartrate 30% 1:1 MEA:HEED 9080 0.05% stannous gluconate 5 l5 Aqueous monoethanolamine solutionhaving 15 weight percent monoethanolaminc.

Aqueous monoethanolamine solution having 30 weight percentmonoethanolamine.

"Incompletely soluble in the test solution.

Aqueous solution containing 30 weight percent ofa 1:1 by weight mixtureof monoethanolamine and N(2-hydroxyethy1)ethylenediamine.

The first corrosion evaluation method used was the determination ofweight losses of metal specimens contained in racks locatedstrategically in the plant streams. The following results were obtainedbefore and after addition of the inhibitor combination:

Location of Corrosion Average Steel Corrosion Rate,mpy

Rack (4 metal specimens Before After each) Inhibitors Added, InhibitorsAdded At the rich to lean amine solution heat exchanger on the lean side2.8 0.8

Rich amine solution down stream of hydraulic turbine 227 0.4

Flow rate of approximately 5 gaL/min. maintained through corrosionracks. Mlls per year as calculated from weight losses after 5 to 22 daysexposure.

The second method of evaluation employed Model CK-2 Corrosometer.Herein, probes of the metals of interest were immersed in the plantsteam and the metal loss was measured by changes in the electricalresistance of the probes. The following readings which were taken over aperiod of several months confirm the efficiency of the inhibitorcombination for both steel and 304 stainless steel.

Corrosometer Readings with Probes in Amine Solution Average IndicatedCorrosion Rate, mpy Type of Before After Metal Inhibitor AdditionInhibitor Addition Mild Steel 550 nil 304 Stainless Steel 1.75 nil It ispointed out that tin inorganic salts where the valence state is +4,e.g., stannic chloride and potassium stannate, often do not appear to beinhibitive.

EXAMPLE 10 Tests equivalent to those conducted in the previous examplewere run using certain nitro-substituted aromatic acids and/or salts andbenzotriazole and the results are set forth herein:

monoethanolamine at indicated temperature for three days.

EXAMPLE 1 1 Certain additional corrosion inhibitors were evaluated underheat transfer conditions as performed in Example 4. The results were asfollows:

Per Cent Protection of Mild Steel Inhibitor, Amine Heat TransferSuspended Solution Plate Panel 0.5% sodium m-nitrobenzoate 15% 89 500.1% nitroterephthalic acid 15% 87 0.5% benzotriazole l 2O 87 Althoughthe invention has been described in its preferred forms with a certaindegree of particularity, it is understood that the present disclosure ofthe preferred forms has been made only by way of example and thatnumerous changes may be resorted to without departing from the spiritand scope of the invention.

What is claimed is:

l. A corrosion inhibited composition consisting essentially of anaqueous alkanolamine solution employed in acid gas removal service andan inhibiting amount of a corrosion inhibitor selected from the classconsisting of:

nitrosubstituted aromatic acids and nitro-substituted acid salts.

2. Composition claimed in claim 1 wherein the corrosion inhibitor issodium m-nitro-benzoate.

3. Composition claimed in claim 1 wherein the corrosion inhibitor issodium 4-nitro-phthaiate.

4. Composition claimed in claim 1 wherein the corrosion inhibitor isnitro-terephthalic acid.

5. Method for inhibiting corrosion of metallic surfaces by an aqueousalkanolamine solution employed in acid gas removal service comprisingadding to said aqueous alkanolamine solution an inhibiting amount of acorrosion inhibitor selected from the group consisting of:

nitro-substituted aromatic acids and nitro-substituted aromatic acidsalts.

6. Method claimed in claim 5 wherein said aqueous alkanolamine solutionis an aqueous monoethanolamine system.

7. Method claimed in claim 5 wherein said corrosion inhibitor is presentin an amount of from about 0.01 to about 2.0 percent by weight basedupon the weight of said aqueous alkanolamine solution.

8. Method claimed in claim 5 wherein the corrosion inhibitor isnitro-terephthalic acid.

9. Method claimed in claim 5 wherein the corrosion inhibitor is sodiumm-nitro-benzoate.

10. Method claimed in claim 5 wherein the corrosion inhibitor is sodium4-nitro-phthalate.

1. A CORROSION INHIBITED COMPOSITION CONSISTING ESENTIALLY OF AN AQUEOUSALKANOLAMINE SOLUTION EMPLOYED IN ACID GAS REMOVAL SERVICE AND ANINHIBITING AMOUNT OF A CORROSION NHIBITOR SELECTED FROM THE CLASSCONSISTING OF: NITRO-SUBSTITUTED AROMATIC ACIDS AND NITRO-SUBSTITUTEDACI SALTS.
 2. Composition claimed in claim 1 wherein the corrosioninhibitor is sodium m-nitro-benzoate.
 3. Composition claimed in claim 1wherein the corrosion inhibitor is sodium 4-nitro-phthalate. 4.Composition claimed in claim 1 wherein the corrosion inhibitor isnitro-terephthalic acid.
 5. Method for inhibiting corrosion of metallicsurfaces by an aqueous alkanolamine solution employed in acid gasremoval service comprising adding to said aqueous alkanolamine solutionan inhibiting amount of a corrosion inhibitor selected from the groupconsisting of: nitro-substituted aromatic acids and nitro-substitutedaromatic acid salts.
 6. Method claimed in claim 5 wherein said aqueousalknaolamine solution is an aqueous monoethanolamine system.
 7. Methodclaimed in claim 5 wherein said corrosion inhibitor is present in anamount of from about 0.01 to about 2.0 percent by weight based upon theweight of said aqueous alkanolamine solution.
 8. Method claimed in claim5 wherein the corrosion inhibitor is nitro-terephthalic acid.
 9. Methodclaimed in claim 5 wherein the corrosion inhibitor is sodiumm-nitro-benzoate.
 10. Method claimed in claim 5 wherein the corrosioninhibitor is sodium 4-nitro-phthalate.